Audio frequency amplifier



March 16, 1943.

N. J. OMAN AUDIO FREQUENCY AMPLIFIER Filed July 8, 1941 uNR RS SL@ m. @su

branch of the plate circuit 55 so that a circuit may be ments of the tubes 20 and 54 and the illament heater winding 55 to the center tap 80 and the lead 28, thence through the choke coil 24 and one of the leads 88, through one-half of the network 39 and the common resistor 4| to ground I0.

The grid circuits of the tubes of the driver ampliiier i8 are returned to the bias supply network 29 .through grid resistors 65 and 88 and supply leads 81 at suitable negative terminals 88 on the network 38, these terminals 'being more negative than the cathode connection points indicated at 89. It will also be noted that suitable suppressor resistors 10 are included in the grid circuits of the driver or cathode follower stage I8.

The plate circuits of the driver stage I3 are held free of signal voltages, and for this' reason the anodes of the parallel connected tubes 20 and 2| and the anodes -18 of the parallel connected tubes 22 and 23 are connected directly with a positive anode supply lead 11 through lters each comprising a series resistor 18 and bypass capacitor 19. Oscillation suppressor resistors 80 are also included in the individual plate connections as indicated. Also, in a transmitter modulator ampliiler, suitable milliammeters for reading the anode current are included in each as indicated at 8|. Each of the coupling choke coils 2l and 25 is provided with an iron core' which may be a core common to both coils as indicated at 85. Associatedl also with each choke coil are a pair of windings coupled thereto, the windings associatedV with the choke coil 25 being indicated at 88 and 81 while those associated with the choke coil 25 are indicated at 88 and 89. These coils are provided with the same number 'of turnsv as the choke coils and are connected with the screen and suppressor grids of the driver amplier stage to cause the suppressor and screen grids to vary in response to signals in the same manner as-the cathodes thereby permitting full tion of the driver tubes.

In the present the cathode circuit of the tubes the winding 88 associated therewith. is connected in circuit with the parallel connected suppressor grids 90, while the winding 81. is connected in circuit with the parallel connected screen grids 9| of these tubes. The screen grid circuit 82 through the winding 81 is connected with a positive supply lead 93, while the suppressorgrid connection 94 is completed through a lead 95 and a negative terminal 96 on the bias supply network 89.

In a similar manner, the suppressor grids 81 of the tubes 22 and 23 are connected through a lead 98 and the choke coil winding 88 with a bias supply lead 99 and a corresponding terminal |00 on the network 39.

The screen grids with a screen grid lead |02, which is, in turn. connected through the winding 89 to the supply lead 98. With this arrangement one winding. that is, the main winding of the choke coil or cathode coupling impedance, on each side of the balanced circuit. supplies bias and signal to the output tubes 25 and 38. A second winding supplies D. C. screen current and the A. C. component of the signal to the screens, while the third winding supplies the D. C. suppressor grid curpentode opera- A heavy' and is used to reduce supplied from the rent and the A. C. component of the signal to the suppressor grids.

As each winding of the choke coil has the same number of turns, full pentode driver operation is permitted in conjunction with a cathode follower coupling circuit by utilizing the auxiliary windings on the cathode chokes to introduce voltages into the suppressor and screen grid leads of the cathode follower tubes, thereby to obtain iixed potentials between the cathode and the suppressor and screen grids.

In order to eliminate undesirable eifects of leakage reactance between the windings of the choke coils, coupling capacitors |05 and |08 are connected be'tween points of equal A. C. or signal potential on the choke coils, in this case between the grid ends of the windings.

The application of the cathode follower principle is useful where the overall feedback, in an ampliiier of the character shown, is relatively distortion and noise and at the same time to supply suillcient energy to the output stage to operate it as a class B ampliiler. driven into the region of heavy grid current.

With a system of the character shown, the bias supply means for the class B amplifier stage may 'be a resistor network 89, to which bleeder current is supplied from any suitable source through the supply leads indicated at |01. This bias supply may also have relatively poor regulation because the gridcurrent on the positive peaks is plate circuit of the driver stage, which is maintained free of signal voltage, in accordance with the present invention. Considering one-half of the balanced driver and output stage, a sine wave of signal voltage Eac, for example, may be impressed on the input grids 5| of the tubes 20' and 2| from the voltage amplifier |2 through the circuit 50. as indicated, and also as indicatedvby'the curve ||0 in Fig. 2. The signal or A. C. voltage on the cathodes of the tubes 20 and 2| vat the cathode end of the choke coil 2| and hence on the input grid circuit 25 of the power output tube 35, is indicated by the curve of Fig. 2. 'Ihis voltage is the resultant of the Eac on the grids of the tubes 20 and 2| and the grid current of the tube 85. If the class B tube 25 did not draw any grid current these twig curves of voltage would be substantially a e.

It is evident that over a portion of the positive half cycle of the input signal. the voltage on the cathodes of the tubes 20 and 2| do not exactly follow the voltage on the grids. This distortion occurs when the tube draws grid current which is indicated by the curve ||2 of'Flg. 2 in proper phase relation to the curves of Fig. 2.

The curve ||2 of Fig. 4 indicates a plate current pulse from the plate circuit of the tubes 2l and 2| added to the normal D. C. plate current and corresponding to the grid current of the tube 35. The departure between the grid and cathode voltages of the tubes 20 and 2| will be such as is required to cause the plate current o f these tubes to duplicate the grid current of the .class B output tube 85. As there is a relaalternating current impedance represented by the output tube grid circuits in shunt with the low impedance choke coil 24 in the cathode circuit of the tubes 20 and 2|, the cathode current will remain substantially constant at the zero signal value over the entire cycle of operation. as indicated by the curve of Fig.

15 5. Thus the grid current for the power output stage is supplied entirely from the plate circuit of the driver stage, which is free from signal voltage. The driver tubes may be of low power rating, of small-size and low cost because of emcient class E operation of these tubes. Also as there is no loading resistor means required for the class B grid circuits to obtain low distortion, the maximum utilization of the driver power with a high degree of efficiency is attained. Furthermore, no driver coupling transformer is required. The latter is ordinarily of relatively large size and high cost as is well known, whereas in the present system a small low cost choke coil is used which carries only the zero signal plate current of the relatively low power driver tubes.

The input or grid impedance of the cathode follower tubes in the driver stage is high because the cathode and screen A. C. or signal potentials are substantially the same as the A. C. or signal grid potentials. Hence, these tubes are not driven positive and the preceding amplifier it may operate as indicated as a simple class A amplifier with a relatively high gain and output impedance without reducing the high frequency response.

it will also be noted that the-impedance from Y the cathodes of the driver stage to ground will be low because of the degenerative effect of the cathode follower connection. Hence stray circuit and tube capacities across this low impedance have little edect on the frequency response of the amplifier except at extremely high frequencies.

It will be noted that the contacts it and itis on the bias supply network may be moved in opposite directions to vary the balance in the biasing of the various grid electrodes and likewise the restance in the network at di may be varied to move all of the grids in the same direction,

I claim as my invention:

l. In an audio frequency inverse feedback amplifier, the combination of a class B output stage, means for driving said output stage to grid current including a cathode follower driver stage coupled thereto and including pentode amplifier tubes having cathode, screen grid, and suppressor grid electrodes, an overall audio frequency inverse feedback circuit for said amplifier includingsaid driver stage thereby to minimize phase shift in the feedback loop of the amplifier, and means in said driver stage for applying substantially equal signal voltages to the cathodes. screen and suppressor grid electrodes for full pentode operation of said stage.

2. Inan audio frequency inverse feedback amplifier, the combination of a. class B output stage, a cathode follower driver stage therefor comprising a pair of pentode amplifier tubes having a balanced output circuit, a cathode circuit choke coil in each half of said output circuit providing an input coupling impedance for said class B output stage for driving' said output stage to grid current, auxiliary windings on each of said choke coils connected with suppressor and screen circuits for said pentode amplifier tubes to apply signal voltage thereto, a voltage amplifier coupled to said driver stage having an audio frequency signalginput circuit, and means providing an overall audio frequency inverse feedback circuit between the output circuit of said first named amplifier stage and input circuit of said voltage amplifier stage thereby to include said cathode follower driver stage in the inverse feedback loop of the amplifier. A

3. In an audio frequency inverse feedback amplifier, means for effecting a maximum degree of stabilized inverse feedback within the frequency range of the amplifier comprising, in combination, a class B audio frequency amplifier stage, an overall audio frequency inverse feedback circuit for said amplifier, and a cathode follower driver stage coupled to said class B cutput stage in the feedback loop oi' the amplier, comprising a pentode tube having an input grid, a suppressor grid, a screen grid, an anode and a cathode, a low impedance choke coil in circuit with the cathode, a signal input connection for the class B stage on said choke coil, an auxiliary winding on said choke coil in circuit with the suppressor grid, a second auxiliary winding on said choke coll in circuit with the screen grid, said windings having substantially the saine number of turns as the choke coil for applying substantially equal signal voltages to said grids and the cathode.

d. In an audio frequency inverse feedback amplier, means for eecting a maximum degree of stabilized inverse feedback within the frequency range of the amplier comprising, in combination, a class B audio frequency amplier stage,

an overall audio frequency inverse feedback circuit for said amplifier, and a cathode follower driver stage coupled to said class E output stage in the feedback loop of the amplifier, comprising a pentode tube having an input grid, a suppressor grid, a screen grid, an anode and a cathode, a low impedance choke coil in circuit with the cathode; a signal input connection for the class B stage on said choke coil, an auxiliary winding on said choke coil in circuit with the suppressor grid, a second auxiliary winding on said choke coil in circuit with the screen grid, said windings having substantially the same number of turns as the choke coil for applying substantially equal signal voltages to said capacity means coupling substantially equal voltage points on said choke coil and windings.

5. In an audio frequency inverse feedback am"- plifier, means for effecting a maximumA degree of stabilized inverse feedback within the frequency range of the amplifier comprising, in combination, a class B audio frequency amplifier stage. an overall audio frequency inverse feedback circuit for said amplifier, and a cathode follower driver stage coupled to said class B output stage in the feedback loop of the amplifier, comprising a pentode tube having an input grid, a suppressor grid, a. screen grid, an anode and a cathode, a low impedance choke coil in circuit with the cathode, a signal input connection for the class B stage on said choke coll, an auxiliary winding on said choke coil in circuit with the suppressor grid, a second auxiliary winding on said choke coil in circuit with the screen grid, said windingshaving substantially the same number of turns as the choke coil for applying substantially equal signal vol ges to said grids and the cathode, and means for maintaining said anode free of signal voltages and for supplying B lstage therefrom.

6. An inverse feedback amplifier having a class B output stage comprising amplifier tubes driven into the positive grid range of operation, an overall inverse feedback connection between the in'- put and output ends of said A amplifier, and a pentode driver stage for said class B output stage grids and the cathode, and

grid current pulses to the class l voltages to the screen and grid electrodes of the pentode driver stage.

7. An inverse feedback amplifier having a class B output stage comprising amplifier tubes driven into the positive grid range of operation, an overall inverse feedback connection between the input and output ends o! said amplier, and a pentode driver stage for said class B output stage having a cathode follower coupling therewith comprising two low impedance cathode choke coils and windings thereon for applying signal voltages to the screen and grid electrodes o! the pentode driver stage. and controlling bias supply network in circuit with said windings and choke coils Iorsupplying biasing potentials to said output and driver stage in predetermined relation.

NIIS J. OMAN. 

